US-A 20070251292 shows a flip integrated circuit flow sensor which has temperature sensors and a heater on top of a substrate, and has a channel for the fluid in the bottom of the substrate, to bring the fluid closer to the sensor. It is also known from U.S. Pat. No. 6,746,932 to provide an area of porous silicon in a substrate to provide better thermal insulation of a temperature sensor on the substrate, so that the sensor is more sensitive to temperature above the substrate.